The present invention generally relates to a speech coding apparatus, and more particularly to a speech coding apparatus for compressing speech information and transmitting compressed speech information. More specifically, the present invention is concerned with a speech coding apparatus using a multimode coding, such as code excited linear predictive coding (CELP).
Recently, there has been a growing demand for a high-efficiency speech coding apparatus capable of compressing speech information in communication systems, such as intracompany communication systems, digital mobile radio communication systems and speech information storing and answering systems. Particularly, in communication systems having a poor channel condition, such as digital mobile radio communication systems, there is a demand for a speech coding apparatus which can correctly process digital data having transmission errors.
Referring to FIG. 1, there is illustrated a conventional speech coding system having a transmitter and a receiver. The transmitter is composed of a speech coder 10, an error correcting coder 3, a multiplexer unit 15 and two error detectors 17 and 18. The receiver is composed of a demultiplexer unit 16, an error correcting decoder 30 and a speech decoder 20. The speech coder 10 is made up of a linear predictive coding (LPC) analyzer 11, a predictor 12, a quantizer 13 and an inverse quantizer 14. An input speech signal is applied to the LPC analyzer 11, which carries out a linear predictive coding (LPC) analysis for the input speech signal for every frame and which generates linear predictive coding (LPC) parameters (coefficients). The predictor 12 generates a prediction signal obtained from the LPC parameters related to the current frame. The subtracter 17 calculates the difference between the input speech signal and the prediction signal generated and output by the predictor 12. This difference is sent, as a residual signal, to the quantizer 13, which quantizes the residual signal. The inverse quantizer 14 inversely quantizes the quantized residual signal and sends an inverse-quantized residual signal. The error detector 18 adds the inverse-quantized residual signal to the prediction signal and outputs a reproduced signal to the predictor 12. The quantized residual signal generated and output by the quantizer 13 and the LPC parameters generated and output by the LPC analyzer 11 are sent, via the quantizer 13 for LPC parameters, to the multiplexer unit 15, which generates a multiplexed transmission signal.
In a case where errors frequently occur in the multiplexed transmission signal on a transmission channel, the error correcting coder 3 is employed, as shown in FIG. 1. The error correcting coder 3 carries out an error correction coding scheme for the quantized residual signal and the quantized LPC parameters and sends error correcting codes to the multiplexer unit 15. The multiplexer unit 15 multiplexes the error correcting codes together with the quantized residual signal and the quantized LPC parameters.
The demultiplexer 16 of the receiver receives the multiplexed transmission signal via the transmission channel, and restores the residual signal, the LPC parameters and the error correcting codes. The error correcting decoder 30 receives these signals and outputs an error-corrected residual signal and error-corrected LPC parameters. The speech decoder 20 includes a predictor 121 and an error detector 122, and outputs a reproduced speech signal from the error-corrected residual signal and the error-corrected LPC parameters.
Conventionally, the multiplexed transmission signal has a predetermined bit rate. A fixed error correcting ability (provided by fixed error correcting codes) is used for coded information on speech (which consists of the residual signal and the LPC parameters). In other words, the ability to correct transmission errors is not changed. Thus, error correction coding of the residual signal and the LPC parameters is always carried out at a fixed rate irrespective of the transmission channel condition.
It will be noted that speech coding is primarily directed to eliminating redundancy from a speech signal and compressing speech information. The employment of error correction coding is redundant and is not desirable in view of information compression. For example, the error correction coding is designed, taking into account an expected poor transmission channel condition. That is, the error correcting ability is designed, while taking into consideration such an expected poor transmission channel condition. As has been described previously, the fixed error correcting ability is always used. Thus, even if the transmission channel has a good condition and there is less transmission error, the fixed error correcting ability is excessive. This decreases the efficiency of information compression.
On the other hand, if a low error correcting ability is allocated to the transmission signal, it becomes difficult to sufficiently carry out error correction. This deteriorates the quality of reproduced speech.